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Journal of Bacteriology, November 2005, p. 7589-7595, Vol. 187, No. 22
0021-9193/05/$08.00+0     doi:10.1128/JB.187.22.7589-7595.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

The Activity Profile of the NhaD-Type Na⁺(Li⁺)/H⁺ Antiporter from the Soda Lake Haloalkaliphile Alkalimonas amylolytica Is Adaptive for the Extreme Environment

Institute of Microbiology, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China,¹ Department of Pharmacology and Biological Chemistry, Mount Sinai School of Medicine, New York, New York 10029,³ Graduate School of Life Sciences, Toyo University, Oura-gun, Gunma 374-0193, Japan,⁴ Graduate School of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China²

Received 4 May 2005/ Accepted 26 August 2005

In extreme alkaliphiles, Na⁺/H⁺ antiporters play a central role in the Na⁺ cycle that supports pH homeostasis, Na⁺ resistance, solute uptake, and motility. Properties of individual antiporters have only been examined in extremely alkaliphilic soil Bacillus spp., whereas the most alkaline natural habitats usually couple high pH with high salinity. Here, studies were conducted on a Na⁺(Li⁺)/H⁺ antiporter, NhaD, from the soda lake haloalkaliphile Alkalimonas amylolytica. The activity profile of A. amylolytica NhaD at different pH values and Na⁺ concentrations reflects its unique natural habitat. In membrane vesicles from antiporter-deficient Escherichia coli EP432 (nhaA nhaB), the pH optimum for NhaD-dependent Na⁺(Li⁺)/H⁺ antiport was at least 9.5, the highest pH that could be tested; no activity was observed at pH 8.5. NhaD supported low Na⁺/H⁺ antiport activity at pH 9.5 that was detectable over a range of Na⁺ concentrations from 10 mM to at least 800 mM, with a 600 mM optimum. Although A. amylolytica nhaD was isolated by complementing the Li⁺ sensitivity of the triple mutant E. coli strain KNabc (nhaA nhaB chaA), sustained propagation of nhaD-bearing plasmids in this strain resulted in a glycine (Gly³²⁷)serine mutation in a putative cytoplasmic loop of the mutant transporter. The altered activity profile of NhaD-G327S appears to be adaptive to the E. coli setting: a much higher activity than wild-type NhaD at Na⁺ concentrations up to 200 mM but lower activity at 400 to 600 mM Na⁺, with a pH optimum and minimal pH for activity lower than those of wild-type NhaD.

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